Exhaust gas purifying catalyst

ABSTRACT

The present invention provides an exhaust gas purifying catalyst that is capable of substantially reducing the degree of deterioration in an exhaust gas purifying capability, which results from dissipation of an absorbent agent. Accordingly, an exhaust gas purifying catalyst, which includes a carrier and a catalyst layer, and to which at least one material selected from a group of alkali metals and alkali earth metals is added as an absorbent agent, is characterized in that acid material with a high affinity with respect to the absorbent agent is mixed in the catalyst layer so as to fix the absorbent agent, and an inhibiting layer is formed between the catalyst layer and the carrier so as to inhibit the movement of the absorbent agent toward the carrier.

FIELD OF THE INVENTION

[0001] This invention relates generally to an exhaust gas purifyingcatalyst, and more particularly to an exhaust gas purifying catalystwith excellent exhaust purifying capability and durability.

BACKGROUND OF THE INVENTION

[0002] A lean combustion type engine such as a lean bum engine and adirect injection type engine is operated at a lean air-fuel ratio, whichis a lower ratio of fuel to air than a stoichiometrical air-fuel ratio,in a predetermined operating range. While the engine is operated at thelean air-fuel ratio, a three-way catalyst cannot satisfactorily purifyNOx (nitrogen oxide) in exhaust gases. Accordingly, it is known that theengine is equipped with an NOx catalyst for absorbing NOx in exhaustgases in an oxide atmosphere, and the NOx absorbed by the NOx catalystis reduced into N2 (nitrogen) in a reduced atmosphere so as to decreasean NOx output into the air. For example, potassium (K) is added as anNOx absorbent agent to the above-mentioned occlusion type lean NOxcatalyst in order to improve an NOx absorbing performance as disclosedin Japanese laid-open Patent Publication No. 9-85093.

[0003] If, however, the NOx catalyst to which the absorbent agent suchas potassium is added is exposed to a high temperature for a long periodof time, the catalyst may crack to cause deterioration in the durabilityof the NOx catalyst.

SUMMARY OF THE INVENTION

[0004] It is therefore an object of the present invention to provide anexhaust gas purifying catalyst that is able to substantially reduce thedeterioration of the exhaust gas purifying performance.

[0005] To attain the above object, the present invention provides anexhaust gas purifying catalyst, which includes a carrier and a catalystlayer, and to which at least one material selected from a group ofalkali metals and alkali earth metals is added as an absorbent agent.Acid material with a high affinity with respect to the absorbent agentis mixed in the catalyst layer, and an inhibiting layer is formedbetween the catalyst layer and the carrier so as to inhibit movement ofthe absorbent agent toward the carrier. Therefore, the acid materialfixes the absorbent agent in the catalyst layer, and the inhibitinglayer inhibits the movement of the absorbent agent from the catalystlayer toward the carrier. This prevents the evaporation of the absorbentagent and the dissipation of the absorbent agent resulting from itsinfiltration into the carrier, and also prevents cracking of thecatalyst caused by the infiltration of the absorbent agent into thecarrier.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The nature of this invention, as well as other objects andadvantages thereof, will be explained with reference to the accompanyingdrawings, in which like reference characters designate the same orsimilar parts throughout the figures and wherein:

[0007]FIG. 1 is a local enlarged sectional view showing a quarter of onecell in a cordierite carrier that is applied to an exhaust gas purifyingcatalyst according to the first embodiment of the present invention;

[0008]FIG. 2 is a conceptual diagram showing a potassium fixing actionthat is executed by a cation exchange ability of zeolite;

[0009]FIG. 3 is a chart showing the affinity of an oxide material withrespect to potassium;

[0010]FIG. 4 is a chart showing the potassium content in a catalystlayer after a heat duration test;

[0011]FIG. 5 is a chart showing the purification efficiency of an NOxcatalyst after a heat duration test;

[0012]FIG. 6 is a local enlarged sectional view showing a quarter of onecell in an exhaust gas purifying catalyst according to a modificationexample of the first embodiment of the present invention;

[0013]FIG. 7 is a local enlarged sectional view showing a quarter of onecell in an exhaust gas purifying catalyst according to the secondembodiment of the present invention; and

[0014]FIG. 8 is a local enlarged sectional view showing a quarter of onecell in an exhaust gas purifying catalyst according to the thirdembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0015] A description will hereunder be given of an exhaust gas purifyingcatalyst according to the first embodiment of the present invention.

[0016] The exhaust gas purifying catalyst according to this embodimentis an NOx catalyst having a porous honeycomb (monolith) cordieritecarrier comprised of many cells. FIG. 1 shows a quarter of one cell inthe cordierite carrier applied to the exhaust purifying catalyst of thisembodiment. The cell in the cordierite carrier 10 is, e.g. square. Forexample, the cordierite carrier 10 is produced by mixing aluminapowders, silica powders and magnesia powders in such a manner that theratio of alumina, silica and magnesia is equal to a cordieritecomposition, then dispersing the mixture of powders with water, formingthe mixture into a honeycomb form, and sintering the honeycomb shapedmixture.

[0017] An inhibiting layer 20 covers the surface of the cordieritecarrier 10, and a catalyst layer 30 is formed on the surface of theinhibiting layer 20. The inhibiting layer 20 is composed mainly ofsilica (SiO₂). In the catalyst layer 30, noble metals such as platinum(Pt), an NOx absorbent agent such as potassium (K) and barium (Ba), andacid material 40 composed of zeolite are finely dispersed and mixed inthe catalyst layer 30.

[0018] For example, the inhibiting layer 20 is formed in the followingmanner. First, a slurry is prepared which includes noble metal such asplatinum (Pt); an absorbent agent formed of alkali metal, alkali earthmetal, etc. such as potassium (K) and barium (Ba); acid material such aszeolite; and the like. The cordierite carrier 10 is then immersed in theslurry and is sintered after drying. Consequently, the inhibiting layer20, which is composed mainly of silica, is formed on the surface of thecordierite carrier 10.

[0019] The catalyst layer 30 is formed on the surface of the inhibitinglayer 20, e.g. in the following manner. First, a slurry is preparedwhich includes noble metals such as platinum, an NOx absorbent agentsuch as potassium and barium, and the acid material 40 composed ofzeolite. The cordierite carrier 10, on which the inhibiting layer 20 isformed in the above-mentioned manner, is immersed in the slurry. Thecordierite carrier 10 is then dried and sintered to thereby form thecatalyst layer 30, in which the noble metals and the absorbent agent aswell as the acid material 40 are mixed, is formed on the inhibitinglayer 20. Typical examples of the absorbent agent are potassium andbarium, but the absorbent agent may be composed of any other substancesinsofar as they are alkali metals or alkali earth metals.

[0020] The NOx catalyst with the above arrangement is placed in a casethrough a cushioning material, and is disposed in an exhaust system of alean burn internal combustion engine. The NOx catalyst absorbs NOx inthe form of nitrite from exhaust gases due to the operation of theabsorbent agent dispersed in the catalyst layer 30, and emits thenitrite from the absorbent agent while the engine is operating at a richair-fuel ratio. The nitrite is then reduced into nitrogen and is emittedfrom the NOx catalyst.

[0021] If the internal combustion engine equipped with theabove-described NOx catalyst is operated for a long period of time, theNOx catalyst is exposed to a high temperature. The conventional NOxcatalyst, in which the catalyst layer 30 with potassium added is merelyformed on the cordierite carrier 10, has such a disadvantage that thepurification performance is deteriorated by the reaction consumption dueto the infiltration of the potassium into the cordierite carrier 10 andthe dissipation of the potassium due to evaporation and the infiltratedpotassium generates a chemical compound in the cordierite carrier 10 tocrack the catalyst.

[0022] To the contrary, the exhaust gas purifying catalyst according tothe present embodiment fixes the absorbent agent such as potassium andbarium included in the catalyst layer 30 due to the cation exchangeability and reactivity of the acid material 40 such as zeolite, and theinhibiting layer 20 composed mainly of silica inhibits the movement ofthe absorbent agent from the catalyst layer 30 toward the cordieritecarrier 10. This prevents the above-mentioned problems such asdissipation of the absorbent agent and cracking of the catalyst.

[0023] The absorbing fixing action and the absorbent agent movementinhibiting action will be described hereinbelow. The zeolite, which is amain component of the acid material 40, is a complex oxide having anexcellent capability to fix the absorbent agent (potassium in thisembodiment) due to its cation exchange ability. The absorbent agent,which moves in the catalyst layer 30, may be ionized under the presenceof high-temperature water moisture. As shown in the conceptual diagramof FIG. 2, the absorbent agent such as potassium ions is fixed due tothe cation exchange ability of acid points in the zeolite. The zeolitehas a large specific surface area with a three-dimensional netstructure, and thus, the absorbent agent is highly diffused on thezeolite. The absorbent agent can be securely fixed.

[0024] The cation exchange ability of the zeolite is in inverseproportion to the ratio of SiO2/A102, and the heat-resisting property isin proportion to this ratio. More specifically, if the ratio ofSiO₂/AlO₂ is small, the acid points are increased with the increase inthe quantity of alumina so that the absorbent agent can be fixed moresecurely. If the ratio of SiO₂/AlO₂ is small, however, alumina isremoved from the zeolite to deteriorate the heat-resisting property ofthe acid material 40. Accordingly, the ratio is preferably determined inview of the absorbent agent fixing action and the durability of the acidmaterial 40. The acid material 40 may be either a natural product or asynthetic product, and more preferably, it has a large specific surfacearea so as to ensure a large physical absorption area. Since titaniumdioxide (TiO₂) achieves the same effect as the zeolite, the titaniumdioxide may be mixed in the catalyst layer 30 instead of zeolite.

[0025] The acid material 40 is preferably composed of Group-IV, Group-Vand Group-VI transition elements or Group-IV, Group-V and Group-VItypical elements (e.g. Si, P, S, V, Cr, As, Nb, Mo, and W) as shown inFIG. 3, and has a high affinity with respect to alkali metal or alkaliearth metal (FIG. 2 shows the affinity with respect to, e.g. potassium).In view of the reactivity with the absorbent agent, the acid material 40is preferably composed of silicon (Si) or tungsten (W) that neverdisturbs the reaction of NOx and the absorbent agent in the case wherethe absorbent agent is composed of potassium.

[0026] On the other hand, the inhibiting layer 20 composed mainly ofsilica has a very high acidity, and the absorbent agent such aspotassium and barium in the catalyst layer 30 is composed of alkalimetal or alkali earth metal. Thus, if the absorbent agent moves from thecatalyst layer 30 toward the inhibiting layer 20, the inhibiting layer20 fixes the absorbent agent irrespective of the fixing action by theacid material 40. This prevents the infiltration of the absorbent agentinto the cordierite carrier 10. The inhibiting layer 20, which achievesthe above-mentioned effects, needs the same requirements as the acidmaterial 40 described with reference to FIG. 3. The inhibiting layer 20may be composed of tungsten instead of silica.

[0027] The movement of the absorbent agent may also be inhibited byforming the inhibiting layer from a material such as titanium dioxide,an alkali metal such as barium, a basic material such as barium oxide(BaO) or the like. In this case, the inhibiting layer has the sameproperty as the absorbent agent, and thus repulses the approachingabsorbent agent to decrease the inductivity of the absorbent agent intothe cordierite carrier 10.

[0028] Further, the movement of the absorbent agent may be inhibited byforming the inhibiting layer 20 of a material such as zeolite having alarge specific surface area, an element chemical compound composedmainly of stable basic material with a high molecular weight such asbarium sulfate (BaSO₄), a material with a small crystal lattice, or thelike. For example, if the inhibiting layer 20 is composed mainly ofzeolite, the absorbent agent is highly diffused in the inhibiting layerwith a large specific surface area. This prevents the infiltration ofthe absorbent agent into the cordierite carrier 10. Moreover, thezeolite has the excellent capability to fix the absorbent agent due toits cation exchange ability as stated above, and thus, the absorbentagent fixing action of the zeolite prevents the infiltration of theabsorbent agent into the cordierite carrier 10.

[0029] With this arrangement, the exhaust gas purifying catalyst of thepresent embodiment prevents the absorbent agent from infiltrating intothe cordierite carrier 10 so as to avoid the reactive consumption andthe evaporation of the absorbent agent, and also prevents cracking ofthe cordierite carrier 10 to improve the durability.

[0030] To confirm the above-described operation of the presentembodiment, the inventors produced an NOx catalyst in which a catalystlayer 30 including potassium as an absorbent agent is formed on thecordierite carrier 10, and found the potassium content in the unused NOxcatalyst with an XRF method (fluorescent X-ray spectroscopic analysismethod). The inventors then found the potassium content in the catalystwhich had been used at a high temperature for a long period of time(e.g. 32 hours at 850° C.), and divided the difference in the potassiumcontent between the unused catalyst and the used catalyst by the initialpotassium content to find the quantity of dissipated potassium. As aresult, the quantity of dissipated potassium in the conventionalcatalyst was found to be between 10% and 50%. According to the testresults, the potassium content in the catalyst layer 30 after the heatduration test was larger than that in the conventional catalyst.

[0031] Further, we compared the NOx purification efficiency after theheat duration test. It was found that the NOx catalyst of the presentembodiment maintained the high NOx purification efficiency irrespectiveof the catalyst temperature.

[0032] On the other hand, the exhaust gases include a slight amount ofHC even when the engine is operating at the lean air-fuel ratio. Sincethe zeolite has the excellent capability to absorb reduced substancessuch as HC, the HC absorbed onto the zeolite facilitates thedecomposition of nitrate and sulfate in the NOx absorbent agent. Morespecifically, even when the engine is operating at the lean air-fuelratio, the zeolite having the HC absorbing ability continuouslydecomposes the nitrate and the sulfate in the NOx absorbent agent byusing a slight amount of HC included in the exhaust gases, therebyrecovering the NOx absorbing capability of the catalyst. It is possibleto use various types of zeolite such as MFI type, Y type, X type,mordenite, ferrierite and β(beta) type. The zeolite, which correspondsto the composition of the exhaust gases, should be selected in view ofthe structural relevancy with respect to the absorbed HC.

[0033] Although sulfate composed of sulfur deteriorates the purificationcapability of the NOx catalyst, the exhaust gas purifying catalyst ofthe present invention is able to diffuse and holds the NOx absorbentagent such as potassium in the catalyst layer 30 to thereby inhibit thegrowth of the sulfate.

[0034] That completes the description of the first embodiment; however,it should be understood that there is no intention to limit theinvention to the specific forms disclosed, but on the contrary, theinvention is to cover all modifications, alternate constructions andequivalents. For example, relatively small particles of the acidmaterial 40 are mixed in the catalyst layer 30 according to the firstembodiment, but relatively large particles or blocks of the acidmaterial 40 may be mixed in the catalyst layer 30.

[0035] There will now be described an exhaust gas purifying catalystaccording to the second embodiment of the present invention.

[0036] This embodiment aims at maintaining the noble metal oxidizingaction in the catalyst layer of the catalyst and the NOxabsorbing/purifying capability of the catalyst at a satisfactory level,and improving the durability of the catalyst. The catalyst of thisembodiment has basically the same structure as the first embodiment(FIG. 1), but is different from the first embodiment in that a secondcatalyst layer 50 is formed on the external surface of a catalyst layer(hereinafter referred to as the first catalyst layer) as shown in FIG.7.

[0037] As shown in FIG. 7, the exhaust gas purifying catalyst comprisesthe cordierite carrier 10, the inhibiting layer 20 which covers thesurface of the cordierite carrier 10, the first catalyst layer 30 whichis formed on the surface of the inhibiting layer 20 and to which theacid material 40 is added, and the second catalyst layer 50. Thestructures of the carrier 10, the inhibiting layer 20 and the firstcatalyst layer 30 are the same as those of the first embodiment, andthus a description thereof will be omitted. The second catalyst layer 50is different from the first catalyst layer 30 in that it does notinclude the acid material 40.

[0038] The exhaust gas purifying catalyst with the above arrangement isproduced by immersing the cordierite carrier 10, on which the inhibitinglayer 20 and the first catalyst layer 30 are sequentially formed, in aslurry including the noble metals and the NOx absorbent agent and thendrying and sintering the cordierite carrier 10 in the same procedure asin the first embodiment.

[0039] As stated previously, the NOx absorbent agent with a highelectron-donating property deteriorates the oxidizing performance of thenoble metals and causes deterioration in the NOx absorbing and purifyingcapability of the catalyst. According to the present embodiment,however, the catalyst layer of the catalyst comprises the first catalystlayer 30 including the acid material 40 and the second catalyst layer 50that is formed on the external surface of the first catalyst layer 30and does not include the acid material 40, so that the NOx absorbentagent in the second catalyst layer 50 can easily move toward the firstcatalyst layer 30. Therefore, the absorbent agent is concentrated in thefirst catalyst layer 50, and the quantity of the absorbent agent in thesecond catalyst layer 50 is decreased. As a result, the oxidizing actionof the noble metals in the second catalyst layer 50 cannot easily bedeteriorated, and the oxidizing action of the noble metals in the entirecatalyst layer and more particularly in the second catalyst layer 50 andthe absorbing and purifying capability of the catalyst can be maintainedat a high level. This inhibits the evaporation of the absorbent agentfrom the second catalyst layer 50, and maintains the absorbingcapability of the catalyst at a satisfactory level. Further, the secondcatalyst layer 50 protects the first catalyst layer 30 and theinhibiting layer 20 from high-temperature exhaust gases. This preventsthe heat degradation of the first catalyst layer 30 and the inhibitinglayer 20 and improves the durability of the catalyst as a whole.

[0040] As is the case with the first embodiment, a heat duration testwas conducted for the exhaust gas purifying catalyst of the presentembodiment. According to the test results, the potassium content in thefirst and second catalyst layers 30, 50 after the heat duration test waslarger than that in the conventional catalyst as indicated by the thirdrectangle from the left in FIG. 4. The NOx purification rate after theheat duration test was improved over the whole range of the catalysttemperature as indicated by a square mark in FIG. 5.

[0041] There will now be described an exhaust gas purifying catalystaccording to the third embodiment of the present invention.

[0042] As is the case with the second embodiment, the third embodimentaims at maintaining the noble metal oxidizing action in the catalystlayer and at a satisfactory level and improving the durability of thecatalyst. The catalyst of this embodiment, however, is different fromthat of the second embodiment in the positions of both catalyst layers.

[0043] As shown in FIG. 8, the exhaust gas purifying catalyst of thisembodiment comprises the cordierite carrier 10, the inhibiting layer 20which covers the surface of the cordierite carrier 10, the secondcatalyst layer 50 which is formed on the surface of the inhibiting layer20 and does not include the acid material 40, and the first catalystlayer 30 which is formed on the surface of the second catalyst layer 50and includes the acid material 40. In short, the second catalyst layer50 is provided between the inhibiting layer 20 and the first catalystlayer 30.

[0044] The exhaust gas purifying catalyst with the above arrangement isproduced in the following manner. The cordierite carrier 10, on whichthe inhibiting layer 20 and is formed, is immersed in a slurry includingthe noble metals and the NOx absorbent agent and then drying andsintering the cordierite carrier 10 to obtain the carrier 1 0, on whichthe inhibiting layer 20 and the second catalyst layer 50 are formed, inthe same procedure as in the first embodiment. The cordierite carrier 10is immersed in a slurry including noble metals, the NOx absorbent agentand the acid material 40 and is then dried and sintered to therebyobtain the exhaust gas purifying catalyst on which the inhibiting layer20, the second catalyst layer 50 and the first catalyst layer 30 areformed.

[0045] In the exhaust gas purifying catalyst that is produced in theabove-mentioned manner, the absorbent agent is concentrated in the firstcatalyst layer 30. Therefore, the oxidizing action of the noble metalsin the second catalyst layer 50 cannot easily be deteriorated, and theoxidizing action of the noble metals in the entire catalyst layer andmore particularly in the second catalyst layer 50 and the absorbing andpurifying capability of the catalyst can be maintained at a high level.According to the present embodiment, if the NOx absorbing and purifyingcapability of the catalyst deteriorates due to the generation of nitrateand sulfate caused by the chemical reaction of the absorbent agent withnitrogen components and sulfur components in the exhaust gases, theair-fuel ratio of the exhaust gas is made rich so as to recover the NOxabsorbing and purifying capability. According to the present embodiment,the absorbent agent is concentrated in the first catalyst layer 30, i.e.on the external surface of the catalyst, and the nitrate and the sulfateare concentrated on the external surface of the catalyst and easily comeinto contact with the exhaust gases. Therefore, even if the air-fuelratio of the exhaust gases is maintained at a rich ratio for a shortperiod of time or if the degree of richness in the air-fuel ratio issmall, the absorbing and purifying capability of the catalyst can berecovered sufficiently. In short, it is possible to inhibit thedeterioration in fuel economy caused by the richness of the exhaust gas.

[0046] According to the results of the heat duration text conducted forthe exhaust (as purifying catalyst of the present embodiment, thepotassium content after the heat duration test was large as indicated bythe fourth rectangle from the left in FIG. 4. The NOx purification rateafter the heat duration test was maintained at a high level as indicatedby an inverse triangle mark in FIG. 5.

[0047] It should be understood, however, that there is no intention tolimit the invention to the first embodiment through the third embodimentdisclosed, but on the contrary the invention is to cover allmodifications, alternate constructions and equivalents falling withinthe spirit and scope of the invention as expressed in the appendedclaims.

[0048] For example, the honeycomb cordierite carrier 10 is employed as aporous carrier according to the first embodiment through the thirdembodiment, but the present invention may also be applied to an exhaustgas purifying catalyst having a carrier that is composed of othermaterial than cordierite. If a metal carrier is used, the infiltrationof the NOx absorbent agent into the carrier causes almost no problemwhereas the evaporation of the absorbent agent and the deterioration inthe exhaust gas purifying capability of the catalyst are prevented. Ifthe honeycomb cordierite carrier is used, a cell in the cordieritecarrier should not necessarily be square but it may be triangular orhexagonal. In the second and third embodiments, the blocks of acidmaterial 40 may be mixed in the first catalyst layer 30 as is the casewith the modified example (FIG. 6) of the first embodiment.

What is claimed is:
 1. An exhaust gas purifying catalyst comprising: acarrier; a catalyst layer; and an inhibiting layer formed between saidcatalyst layer and said carrier; wherein at least one material selectedfrom a group consisting of alkali metals and alkali earth metals isadded to the catalyst layer as an absorbent agent wherein acid materialwith a high affinity with respect to said absorbent agent is mixed insaid catalyst layer; and wherein the inhibiting layer inhibits movementof said absorbent agent toward said carrier.
 2. An exhaust gas purifyingcatalyst according to claim 1, further comprising: a second catalystlayer formed on an external surface of said catalyst layer; and whereinat least one material selected from a group consisting of alkali metalsand alkali earth metals is added to the second catalyst layer as anabsorbent agent.
 3. An exhaust gas purifying catalyst according to claim1, further comprising: a second catalyst layer formed between saidcatalyst layer and said inhibiting layer; and wherein at least onematerial selected from a group consisting of alkali metals and alkaliearth metals is added to the second catalyst layer as an absorbentagent.
 4. An exhaust gas purifying catalyst according to claim 1,wherein said acid material or said inhibiting layer is composed of atleast one of the following materials: an acid oxide including at leastone acid substance selected from GroupIV, Group-V and Group-VItransition elements and Group-IV, Group-V and Group-VI typical elements;a complex oxide including said at least one acid substance; a materialthat never inhibits reaction of a nitrogen oxide and said absorbentagent; and a material that absorbs a reduced substance.
 5. An exhaustgas purifying catalyst according to claim 1, wherein said acid materialis composed of zeolite; and wherein said inhibiting layer is composed ofsilica (SiO2).
 6. An exhaust gas purifying catalyst according to claim1, wherein said inhibiting layer is composed of at least one of thefollowing materials: a layer with a high acidity, a layer with a largespecific surface area, a layer with a small crystal lattice, a layercomposed of an element compound, and a layer with a high alkalinity. 7.An exhaust gas purifying catalyst according to claim 1, wherein saidabsorbent agent includes potassium (K); and wherein said carrier iscomposed of a porous carrier.
 8. A method of manufacturing an exhaustgas purifying catalyst comprising: forming a carrier; forming aninhibiting layer on the carrier; and forming a catalyst layer on theinhibiting layer; wherein at least one material selected from a groupconsisting of alkali metals and alkali earth metals is added to thecatalyst layer as an absorbent agent wherein acid material with a highaffinity with respect to said absorbent agent is mixed in said catalystlayer; and wherein the inhibiting layer inhibits movement of saidabsorbent agent toward said carrier.
 9. A method of manufacturing anexhaust gas purifying catalyst according to claim 8, further comprising:forming a second catalyst layer on an external surface of said catalystlayer; and wherein at least one material selected from a groupconsisting of alkali metals and alkali earth metals is added to thesecond catalyst layer as an absorbent agent.
 10. A method ofmanufacturing an exhaust gas purifying catalyst according to claim 8,further comprising: forming a second catalyst layer between saidcatalyst layer and said inhibiting layer; and wherein at least onematerial selected from a group consisting of alkali metals and alkaliearth metals is added to the second catalyst layer as an absorbentagent.
 11. A method of manufacturing an exhaust gas purifying catalystaccording to claim 8, wherein said acid material or said inhibitinglayer is composed of at least one of the following materials: an acidoxide including at least one acid substance selected from GroupIV,Group-V and Group-VI transition elements and Group-IV, Group-V andGroup-VI typical elements; a complex oxide including said at least oneacid substance; a material that never inhibits reaction of a nitrogenoxide and said absorbent agent; and a material that absorbs a reducedsubstance.
 12. A method of manufacturing an exhaust gas purifyingcatalyst according to claim 8, wherein said acid material is composed ofzeolite; and wherein said inhibiting layer is composed of silica (SiO₂).13. A method of manufacturing an exhaust gas purifying catalystaccording to claim 8, wherein said inhibiting layer is composed of atleast one of the following materials: a layer with a high acidity, alayer with a large specific surface area, a layer with a small crystallattice, a layer composed of an element compound, and a layer with ahigh alkalinity.
 14. A method of manufacturing an exhaust gas purifyingcatalyst according to claim 8, wherein said absorbent agent includespotassium (K); and wherein said carrier is composed of a porous carrier.